Very easy to do, but I'll spell it out, since you're probably unfamiliar with the procedure.
1. Bring the pic to the Desktop. Open Mail, and you'll see the pic you want to make a screensaver, and drag and drop the image onto your computer desktop.
2. Bring the pic to Documents. You want to make sure your pic is safe and accessible, so go to Finder/Documents, and drop the image into the Documents Folder.
3. Go to System Preferences. You can close Mail, if you haven't already, and go to System Preferences/Desk & Screensaver/Desktop.
4. Find the Image. On Desktop, under Folders, you'll see the image you want to use as a screensaver.
5. Install the image. Tap on the image you want to make your desktop, and it will appear on screen.
If you have any questions, respond to this post, and I'll try to clear it up. Post here to make sure everything worked out!